If you have either looked into buying or already own a server, you've probably head of a RAID array. A RAID array is system that uses multiple hard disk to create some form of fault tolerance when connected to a server. Basically it acts as a safety net for if a drive fails. It helps keep you up and running while also protecting you from data loss in most cases.
Servers typically have a RAID controller installed which is software that controls the disk array.
There are several different levels of RAID protection and which one you choose depends on if you are looking for a performance boost or if you are looking for fault tolerance. Your choice of RAID also impacts which type of drives you can use.
The common RAID levels are RAID 0, 1, 5 ,6 and 10. Each has their own upsides and downsides which you can read about here.
RAID 5 is the most common RAID configuration for business server and enterprise NAS devices because it provides better performance than mirroring, as well as fault tolerance. If a disk fails the data is recreated automatically. Another awesome feature of RAID 5 is that drives are 'hot-swappable' meaning that you can switch out a failed drive without having to shutdown the server and interrupt work. The only drawback is that because of all the protections built into it, write operations can experience a lag.